Just ran an experiment. Calm winds, very quiet out on my backyard overlooking a greenbelt and creek. (We call them that because only the creek bottoms are really very green in Dallas).
Girlfriend positioned 25 feet away, in front and to my left, back to me (at bearing ~ 300 degree (NNW), firing off hand standing.
Fired one round out into the grass of the greenbelt, 40 yds range. Wait and reload.
She rates the loudest as the Whisper .177, with the other two both quieter and almost same level. The Slavia CZ634 (.177) was quietest, and the B-26 .22 second.
From shooters viewpoint the same ranking, with the Slavia having a shorter, sharper ring during firing. The Whisper had been treated (today) with a big shot of "Great Stuff" filling the stock. It sounded some better than before to me, but it had a loud "thonk" that reverb'd from the hollow plastic stock when new (t has a high power spring mod tune).
Gene, I shot three ten shot groups earlier this aft. with the Slavia after remounting (and using Loctite 87 high strength along the rails) re-zeroing the scope, and am getting very good patterns. This is just a great little rifle, it shoots better every time than my B-26 and let's not talk about the Whisper. What a sweet gun, and that Bushnell Trophy 4-12 x 40 is a perfect match. Set at about 8-10X, it is better than my best binoculars.